1. Field of the Invention
This invention relates to a process for effecting isomerization of paraffins, including normal paraffins and/or cycloparaffins, in the presence of hydrogen and a catalyst comprising a highly siliceous porous zeolite crystalline material of controlled enhanced acid activity characterized by a silica/alumina mole ratio of greater than about 12 and a constraint index, hereinafter defined, within the approximate range of 1 to 12, which zeolite has intimately combined therewith a minor proportion of a Group VIII metal.
2. Description of the Prior Art
It has heretofore been known that the isomerization of paraffins, such as n-hexane, to equilibrium mixtures of branched chain isomers, substantially increases the octane rating of the paraffin hydrocarbons. Prior art processes dealing with paraffin isomerization include a liquid-phase process utilizing a catalyst containing an aluminum chloride dissolved in antimony trichloride, which, together with HCl formed during the reaction, are corrosive. Another process, referred to as the Penex process, which has been used for isomerizing pentane and hexane fractions from refinery naphthas and natural gasolines in the presence of hydrogen and a platinum containing catalyst requires the continuous addition to the feed of an organic chloride which also produces HCl. Another process involves reaction of a pentane/hexane feed in the presence of hydrogen and a catalyst containing platinum highly dispersed on hydrogen mordenite at a pressure of 150-350 psi and a temperature of 400 to 550 degrees F. with a maximum water level in the feed of 50 ppm. The above processes have been costly to operate because of extensive corrosive effects attributable to the use of highly acidic catalysts or by-products arising from use of such catalyst, thereby requiring expensive alloy equipment. Moreover, moisture and high molecular weigtt hydrocarbons usually present as contaminants in the charge stock cause deterioration of the catalyst and necessitate frequent replacement thereof. Another process which has been carried out at a higher temperature of 700 to 800 degrees F. utilizes a catalyst such as platinum on a silica-alumina base in the presence of hydrogen. At the high temperatures required, the equilibrium mixture of isomers is such that substantial recycling of a portion of the paraffin feed is necessary to obtain the desired improvement in octane rating.
Prior patents dealing with paraffin isomerization include U.S. Pat. No. 3,432,568 which describes hydroisomerization of saturated aliphatic and cyclic hydrocarbons by contacting with a mixed dual-functional catalyst comprising hydrogen mordenite and a dehydrogenation component supported on a thermally stable carrier. U.S. Pat. No. 3,301,917 relates to hydroisomerization of paraffinic hydrocarbons in the presence of a mixed catalyst consisting essentially of an acid aluminosilicate portion and a hydrogenation component of a platinum metal supported on a thermally stable carrier. U.S. Pat. No. 3,673,267 describes process for isomerization of parafffinic hydrocarbons under isomerizing conditions and in the presence of hydrogen with a catalyst of hydrogen mordenite having a silica to alumina mole ratio between about 20:1 and about 60:1, having associated therewith a metal of Group VIII, Group VIB or Group IB. U.S. Pat. No. 3,370,099 is concerned with hydroisomerization of isomerizable hydrocarbons in the presence of a catalyst composition of an aluminosilicate containing at least 0.5 equivalents per gram atom of aluminum of metal cations of which at least some and generally 50 percent or more of the total equivalents are cations of a rare earth metal. U.S. Pat. No. 3,702,886 describes crystalline zeolite ZSM-5 and indicates that the same may be used as a catalyst for hydroisomerization of normal paraffins when provided with a hydrogenation component, such as platinum.